Melatonin salvages lead-induced neuro-cognitive shutdown, anxiety, and depressive-like symptoms via oxido-inflammatory and cholinergic mechanisms

Abstract

Introduction: Lead is the most used nonphysiological neurotoxic heavy metal in the world that has been indicated to interfere with the cognitive and noncognitive processes via numerous mechanisms. The neuroprotective effect of melatonin is well known, but the effect of its interaction with lead in the brain remains inconclusive.

Objective: To assess the therapeutic role of melatonin on cognitive deficit, anxiety and depressive-like symptoms in matured male Wistar rats exposed to a subchronic lead chloride (PbCl₂ ).

Methods: Twenty male Wistar rats were blindly randomized into four groups (n = 5/group): group 1 to 4 underwent intragastric administration of physiological saline (10 ml/kg; vehicle), PbCl₂ (50 mg/kg), melatonin (10 mg/kg) and PbCl₂ + melatonin respectively for a period of 4 weeks during which neurobehavioral data were extracted, followed by neurochemical and histopathological evaluations.

Results: Exposure to PbCl₂ reduced cognitive performance by increasing the escape latency and average proximity to the platform zone border, decreasing average path length in the platform zone, cognitive score, and time spent in probing. It raised the thigmotaxis percentage, time spent in rearing, number of pellet-like feces, and time spent in the dark compartment of a bright/dark box which are predictors of anxiety. It also induced depressive-like behavior as immobility time was enhanced. PbCl₂ deranged neurochemicals; malondialdehyde, interlukin-1β, and tumor necrotic factor-α were increased while superoxide dismutase and acetylcholinesterase were decreased without remarkable alteration in reduced glutathione and nitric oxide. Administration of PbCl₂ further disrupted neuronal settings of hippocampal proper and dentate gyrus. In contrast, the supplementation of melatonin reversed all the neurological consequences of PbCl₂ neurotoxicity by eliciting its properties against oxidative and nonoxidative action of PbCl₂ .

Conclusion: These findings suggest that melatonin down-regulates neurotoxicant interplays in the brain systems. Therefore, this study suggests the use of melatonin as an adjuvant therapy in neuropathological disorders/dysfunctions.

Keywords: anxiety; cholinergic disturbance; cognition; depression; lead-toxicity; melatonin; neuro-inflammation; neurogenesis; oxidative stress.

FIGURE 1

Experimental timeline and design

FIGURE 2

Representative results for the Morris water maze and ‘bright and dark box’ tasks illustrating the multitasking role of melatonin on lead‐induced cognitive deficit. The following parameters were accessed: In the MWM—Escape latency (a) showing the acquisition trial (days 1–4); time spent in the platform (b); path length in the platform zone (c); average proximity to the platform zone border (d); escape latency for the short (e) and long‐term memory (f); and cognitive score (g) during the probe trial on the 5^th day. In the BDB—time spent in probing (h) was evaluated on the test day. PbCl₂, lead chloride; MEL, melatonin; n = 5/group, ^*significantly different from the control group at p ≤ .05, ^#significantly different from PbCl₂ (50 mg/kg) group at p ≤ .05

FIGURE 3

Representative results for the bright and dark box and Morris water maze tasks showing the reversal effects of melatonin on lead‐induced anxiety‐like behavior. The following parameters were accessed: In BDB—time spent in bright and dark compartments (a) and number of pellet‐like feces (b). In MWM—thigmotaxis (c) and time spent in rearing (d) during the acquisition trial (day 2) were all accessed. Micrographs depicting the swimming trajectories of the rats during the evaluation of anxiety‐like responses and cognitive processes (acquisition, consolidation or retention) when platform was in the pool on the 2nd, 4th, and 5th day for the task (e). Values are expressed as mean ± SEM. Data were analyzed using analysis of variance and Tukey post hoc test for multiple comparisons. n = 5/group. ^*significantly different from the control at p ≤ .05, ^#significantly different from PbCl₂ (50 mg/kg) group at p ≤ .05. PbCl₂; lead chloride, MEL; melatonin, blue and red circles indicate the starting and end points in MWM respectively

FIGURE 4

Representative results for the tail suspension task illustrating the effect of melatonin on lead‐induced depressive‐like symptom—immobility time. Values are expressed as mean ± SEM. Data were analysed using analysis of variance and Tukey post hoc test for multiple comparisons. n = 5/group. ^*Significantly different from the control at p ≤ .05, ^#significantly different from PbCl₂ (50 mg/kg) group at p ≤ .05. PbCl₂; lead chloride, MEL; melatonin

FIGURE 5

Representative results showing the reversal effects of melatonin on lead induced neurochemical derangement in rats. The following parameters were accessed: MDA (a), SOD (b), GSH (c), NO (d), IL‐1β (e), TNF‐α (f) and AChE (g). Values are expressed as mean ± SEM. Data were analyzed using analysis of variance and Tukey post hoc test for multiple comparisons. n = 5/group. ^*Significantly different from the control at p ≤ .05, ^#significantly different from PbCl₂ (50 mg/kg) group at p ≤ .05. PbCl₂; lead chloride, MEL; melatonin, MDA; malondialdehyde, SOD; superoxide dismutase, GSH; reduced glutathione, NO; nitric oxide, TNF‐α; tumor necrotic factor‐α, IL‐1β; intelukin‐1β, AChE; acetylcholinesterase

FIGURE 6

Representative results depicting hematoxylin and eosin (H&E) staining and quantitative neuronal cell count in the hippocampal regions of CA1, CA3, dentate gyrus and suprapyramidal blade of dentate gyrus. Melatonin reversed lead‐induced histopathological distortion in CA1 (a), CA3 (b), DG (c) and SPB (d). For quantitative analysis, melatonin ameliorates neuronal cell count decline in the hippocampal subfields induced by lead (e). The slides (n = 3) were viewed by light microscopy; magnification: × 400; scale bar = 50 μm. Values are expressed as mean ± SEM. Data were analyzed using analysis of variance and Tukey post hoc test for multiple comparisons. n = 3/group. ^*Significantly different from the control at p ≤ .05, ^#significantly different from PbCl₂ (50 mg/kg) group at p ≤ .05. PbCl₂; lead chloride, MEL; melatonin, CA1; CA3; DG; SPB; blue line; pyramidal cell, yellow line; vesicular cell, green line; neuronal degeneration (vacuolization and atrophy), black line; granular cell

FIGURE 7

Graphical abstract illustrating the probable pathways through which melatonin supplementation targeted lead‐induced neuro‐cognitive shutdown, anxiety and depressive‐like symptoms